This invention relates to a process for the solvent refining of coal wherein coal is liquefied by subjecting it to a hydrogen donor solvent (hereinafter referred to as "solvent") in the presence of a hydrogen-rich gas at elevated temperatures and pressures. This process is referred to in the art as SRC-I, solvent refined coal having the acronym "SRC". In this process, following solvation, the products are separated into gaseous material, distillate fractions and vacuum distillation bottoms. The vacuum distillation bottoms, which contain entrained mineral matter and unconverted coal macerals, are separated in a deashing step. From the solids removal step there is recovered a stream of coal products which are free of ash minerals and unconverted coal and which are essentially low in sulfur content, such that this material is ideally suited for combustion in environmentally acceptable operations.
In the operation of a coal liquefaction complex for the process of the above-indicated type, the dissolving section must be capable of generating sufficient process solvents to meet the solvent demands of the plant. Not only must adequate supplies be met, but also the quality of this solvent must be kept at a level such that the process will continue to operate.
The SRC-I pilot plants at Wilsonville, Ala. and Fort Lewis, Wash. have been operated only with a single coal liquefaction reactor (also known as the dissolver) preceded by a preheater. The coal liquefaction reactions take place to some extent in both of these vessels. A slurry of coal in recycled solvent under hydrogen pressure is passed through the preheater, where its temperature is raised from ambient to temperatures up to 800.degree. F. For a typical dissolver outlet temperature of 825.degree. F., the preheater outlet temperature would be about 775.degree. F. The residence time in the preheater is about five minutes. About 85% of the raw coal is dissolved at the preheater exit, but other reactions (desulfurization, solvent production, etc.) do not occur to any major extent in the preheater. The heated slurry then passes to the dissolver, where the bulk of the other liquefaction reactions (desulfurization, solvent production, solvent rehydrogenation, etc.) take place.